This invention relates to the field of electrical signal delay lines wherein signal delay is accomplished by the propagating of a radio frequency signal through a ferrimagnetic film such as a magnetic garnet including the electrical-to-magnetic transducers used with such delay lines.
Magnetostatic wave delay lines are potentially useful in phased-array radar antennas because of the wide instantaneous bandwidth obtained when time delay rather than phase shift is used for beam steering. For example, most present-day phased array radar antennas use 0 to 2.pi. phase shifters for beam steering and thereby achieve a beam angle of EQU .theta.=sin.sup.-1 (.psi.c/2.pi.sf), (1)
an angle which is frequency dependent. In Equation (1) .psi. represents the phase shift increment between elements, c is the velocity of light, f is the frequency, and s is the spacing between antenna elements. It is the relationship of the beam angle (.theta.) with frequency which results in the narrow, instantaneous bandwidth of such present-day phased array antennas. If, however, variable time delay devices such as a magnetostatic delay line are used to steer the antenna, the beam steering angle is EQU .theta.=sin.sup.-1 (tc/s), (2)
an angle which is independent of frequency. In Equation (2) t represents the incremental time delay between array elements and the other parameters are as described for equation (1). Thus, using time delay beam steering, the instantaneous bandwidth of the antenna is only limited by the bandwidth of the delay lines or other microwave components and will typically be in the range of 300 to 1000 MHz. The time delay variation required for a particular antenna element depends upon the antenna structure and is typically .+-.10 ns for a 12 foot aperture. This relationship is discussed in detail in books on radar systems design.
Adjustable delay lines, whose characteristics do not vary with frequency can be obtained by combining an "up-chirp" delay line with a "down-chirp" delay line so that the achieved total delay is constant with frequency. An "up-chirp" delay line has a group delay which increases linearly with frequency. Delay adjustment is therefore achieved by changing the center frequency of one of the delay lines. This frequency change can be achieved for example by varying the strength of the applied magnetic bias field as is described in the copending patent application of K. K. Jin, docket number AF 15274 Ser. No. 06/664,193, filed Oct. 24, 1984.
Dispersive magnetostatic wave (MSW) delay lines also find use in broadband compressive radio receivers for electronic warfare applications. In a compressive receiver, signals within the system bandwidth are mixed with a linear FM chirp, i.e., a signal whose frequency varies linearly with time, and are passed into a dispersive delay line whose delay variation with frequency matches that of the FM chirp. The output of the delay line then effectively provides the Fourier transform of the input signal. A device of this type is actually a very high-speed, boardband spectrum analyzer. Since MSW delay lines have bandwidths in the 500 MHz to 1 GHz range, wide frequency ranges can be covered with a few devices.
Practical uses of the magnetostatic delay line require that the dispersive delay-frequency characteristics of the device be predictable according to some mathematical relationship. Preferably this mathematical relaionship should be a linear first-order equation wherein frequency and delay are correlated by a straight line graphical relationship.
Several MSW delay line techniques have been developed which result in an approximately linear variation of delay with frequency. However, in each of these techniques there is always present a predictable delay error of deviation from the desired linear delay variation with frequency. In particular, two simple delay line techniques have been demonstrated which each show such a linear variation of delay with frequency over a 1 GHz bandwidth, however, each of these techniques also demonstrates a slow "W" shaped delay error characteristic. One of these techniques involves a ground plane spaced from the Yttrium Iron Garnet (YIG) magnetic film of the delay line element a plane spaced by a distance equal to the film thickness. The second technique involves two YIG films of generally equal thickness spaced apart by a non-magnetic layer. These delay lines represent structurally simple techniques, however their usefulness is limited by residual phase error--even though this phase error is predictable and can in principle be compensated for. In the present document a technique is described which may be used in conjunction with these simple delay line structures in order to compensate for the delay error.
The patent art includes several examples of magnetostatic delay line devices; these examples include a first patent of Jean P. Castera, U.S. Pat. No. 4,341,998 which concerns a magnetometer apparatus fabricated with a magnetostatic device that is connected into an oscillator circuit. One view of the Castera magnetometer, in FIG. 4, shows tranducers with multiple elements that are excited by a common signal. This FIG. 4 device employs resonant cavities in the form of grooves but the employed transducers are devoid of different element types and locations used in combination.
The patent of Gerard Volluet, U.S. Pat. No. 4,316,162, describes a magnetostatic wave device, such as a delay line, which has transmitting and receiving transducer elements in the form of thread-like filamentary electrodes. The Volluet apparatus includes magnetic wave attenuating regions formed in the ferrimagnetic garnet magnetic layer by an abrading technique such as sand blasting. The Volluet patent discloses only the simple single filament transducer element and is principally concerned with reduction of interference reflections within the magnetic garnet material on which the transducers are mounted.
A second patent of J. P. Castera et al U.S. Pat. No. 4,314,214, discloses a magnetostatic wave device which may be composed of a ferrimagnetic layer of yttrium iron garnet that is subjected to a polarizing magnetic field. On the Castera garnet layer are mounted transmitting and receiving transducer elements and an exchange structure which serves to receive and retransmit signals originating in the transmitting transducer. The exchange structure is formed by a plurality of transducer elements, spaced by less than half a magnetostatic wavelength, which are connected to form a complete electrical circuit. The exchange structure of the Castera patent is used for wide band signal coupling, a magic T device and for several multi-plane signal coupling arrangements. The Castera invention is concerned with re-transmission of an original transducer signal and with the optimizing of device performance using this retransmitted signal in combination with quarter-wave displacement of the exchange structure elements and tuning of the circuit elements. One transducer disclosed in the Castera patent, the transducer 27 in FIG. 8 includes multiple elements of unequal length which are fed by a single source of signal. None of the embodiments of the Castera patent, however, are concerned with a combination of bar and interdigital transducer elements in a magnetostatic wave device.
Another example of prior art magnetostatic wave device is found in the patent of Ralph W. Patterson et al, U.S. Pat. No. 4,199,737 which includes a common inventor with the present invention but is assigned to a different assignee than the present patent. The Patterson patent concerns a magnetostatic wave device having interdigital transducer fingers fed from a common microwave source and arranged to shape the device frequency response and delay vs. frequency characteristics. FIG. 22 and the text at column 6 line 55 in the Patterson patent described a transducer arrangement employing non-uniform spacing between adjacent fingers with the spacing arranged to compensate for inherent time delay variations with changing frequency.